Circadian oscillators play a critical role in the regulation of visual system function, as visual sensitivity depends on the time of day, irrespective of the level of illumination. The research will examine the regulation and physiological role of a novel Ca2+ permeable cationic channel, known as I-LOT, recently discovered in cultured vertebrate photoreceptors from the pineal gland and retina of the chick. In pineal cells, the activity of this ion channel is controlled by the photoreceptor circadian oscillator such that it is only active in the nighttime, and not during the daytime. This is true even in cells that are maintained in constant-dark conditions. The applicant hypothesizes that I-LOT plays a central role in the regulation of melatonin secretion and other aspects of retinal photoreceptor physiology. Experiments are proposed to study the regulation of I-LOT by circadian oscillators, photic stimuli, and neurohormones in chicken retinal photoreceptors. Specific studies will characterize biophysical aspects of Ca2+ permeation and blockade of I-LOT based on the critical role that Ca2+ plays in regulation of photoreceptor function. Finally, the applicant proposes to attempt to identify molecular factors, such as protein kinases and phosphatases, and protein processing events, that regulate the functional activity of I-LOT channels. A component of his research will entail attempts to functionally express I-LOT channels in Xenopus oocytes by injection of mRNA extracted from chick pineal cells at different times of day. Circadian rhythms may also play an important role in retinal disfunction. It has been proposed that circadian rhythms provide a common pathway leading to degenerative responses to constant light, vitamin A deficiency, and certain forms of retinitis pigmentosa. Therefore it is important to understand the mechanisms whereby the circadian oscillator controls the cellular physiology of the photoreceptor. The fact the I-LOT is Ca2+-permeable, and because Ca2+ plays a central role in regulating cell death under a variety of conditions, makes the characterization of this ionic channel a problem of particular importance. This is because alterations in the gating of I-LOT , or the structure of I-LOT, could cause profound alterations in intracellular Ca2+ homeostasis leading to alterations in photoreceptor responses and even retinal degeneration.